Experimental investigation of the effect of transpiration cooling on second mode instabilities in a hypersonic boundary layer

Abstract

The influence of localized nitrogen transpiration on second mode instabilities in a hypersonic boundary layer is experimentally investigated. The study is conducted using a 7 • half-angle cone with a length of 1100 mm and small nose bluntness at 0 • angle-of-attack. Transpiration is realized through a porous Carbon/Carbon patch of 44 × 82 mm located near the expected boundary layer transition onset location. Transpiration mass flow rates in the range of 0.05-1% of the equivalent boundary layer edge mass flow rate were used. Experiments were conducted in the High Enthalpy Shock Tunnel Göttingen (HEG) at total enthalpies around 3 MJ/kg and unit Reynolds numbers in the range of 1.4 ⋅ 10 6 to 6.4 ⋅ 10 6 m −1 . Measurements were conducted by means of coaxial thermocouples, Atomic Layer Thermopiles (ALTP), pressure transducers and high-speed schlieren. The present study shows that the most amplified second mode frequencies were shifted to lower values as nitrogen is transpired into the boundary layer. In some cases the instability amplitudes were found to be significantly reduced. The observed frequency reduction was verified to correlate with the change of the relative sonic line height in the boundary layer. The amplitude damping was observed to occur only until the most amplified frequencies were reduced to around 50% of their undisturbed values. When transpiration within this limit was performed shortly upstream of the natural boundary layer transition onset, a transition delay of approximately 17% could be observed.

Effects of wall transpiration on the supersonic boundary-layer oblique-type transition

“…Their experiment showed that alcohol can achieve the highest cooling efficiency with a fixed injection rate. In a high enthalpy shock tunnel, Camillo et al [7] studied the hypersonic boundary-layer second-mode stability under the wall transpiration of nitrogen gases. The study revealed IOP Publishing doi:10.1088/1742-6596/2764/1/012029 2 that the boundary-layer transition can be delayed under some specific situations.…”

Displacement of hypersonic boundary layer instability and turbulence through transpiration cooling

“…[4][5][6] In these facilities, researchers commonly utilize the connection of turbulence with aerodynamic friction and hence heat transfer 7 to determine the transition location. 8 Techniques such as infrared thermography, 9,10 thermocouples, 8,11 thin film gauges, 12,13 atomic layer thermopiles (ALTP), 14,15 and temperature sensitive paint 16 are used in the field to infer instability data or laminar-turbulent transition from surface heat flux recordings. However, when a cooling technique is present, surface instruments are evidently subject to coolant and boundary layer gas simultaneously and may therefore be unable to accurately resolve turbulent heating.…”

“…Two more recent studies conducted on cones with gas injection both experimentally inferred transition from surface thermocouple data. 14,31,32 For Jewell et al, 11,31 injection occurred close to the apex, and transition was determined via an intermittency method. 31,33 In contrast, Camillo et al 14 delivered coolant to the boundary layer via a narrow porous injection segment in the direct vicinity of transition on their cone model.…”

“…14,31,32 For Jewell et al, 11,31 injection occurred close to the apex, and transition was determined via an intermittency method. 31,33 In contrast, Camillo et al 14 delivered coolant to the boundary layer via a narrow porous injection segment in the direct vicinity of transition on their cone model. They acknowledge the problem of having cooling and transition present in the same area while attempting to capture a heat transfer increase due to beginning turbulence.…”

“…Their method to infer transition onset is based on comparing theoretically obtained laminar heat flux levels to averaged thermocouple data. 14 Work by Kerth et al 34 has dealt with a similar gas injection scenario, wherein transition was inferred from schlieren images and thin film gauges. Hereby, transition took place far away from the injection location.…”